Metal oxides, particularly silica, have been incorporated into certain surface coating materials in order to achieve the impression of “mattness.” A wet film applied to a substrate is initially held flat by the forces of surface tension, thereby resulting in a glossy surface. As the film dries and cures, the increasing viscoelasticity associated with the sol-gel transition hinders the movement of particles into the film, and the surface deforms to accommodate the matting agent particles. This roughness is maintained in the solidified film, which then is characterized by a matt finish. While the matting effect attributable to certain metal oxides produces a desirable surface appearance for some applications, a surface coating composition desirably possesses additional properties, such as corrosion resistance, in order to enhance its utility for a variety of applications.
Corrosion resistance is a characteristic of a composition that resists damage (e.g., bubbling, cracking, and staining) after wetting by or absorption of water and/or other materials (e.g., liquids) or after exposure to adverse conditions that would tend to cause the coated substrate to lose its original function. Prior attempts at rendering a surface coating composition more corrosion resistant have used the incorporation of certain metal oxides, particularly treated fumed silica, into the composition.
Silica, an inorganic material having silicon dioxide (SiO2) as a basic structural unit, is useful in a wide variety of commercial applications. Silica exists in a variety of molecular forms, which include, for example, monomers, dimers, oligomers, cyclic forms, and polymers. In addition, silica can be amorphous, crystalline, hydrated, solvated, or dry, and can exist in a variety of particulate and aggregated states.
Amorphous silica can be formed by molecular precipitation, for example, by cooling a supersaturated solution, concentrating an undersaturated solution, or by careful hydrolysis of a solution of a labile silica precursor, such as a SiCl4, esters of silica, Si(OR)4, and the like, to provide a supersaturated solution of Si(OH)4, from which precipitates amorphous silica.
Pyrogenic, or “fumed silica,” which typically has a particle size from about 2-20 nm, is formed from the vapor phase. For example, silica (usually sand) can be vaporized at about 2000° C. and cooled to form anhydrous amorphous silica particles. Alternatively, silica can be sublimed at about 1500° C. in the presence of a reducing agent (e.g., coke) to form SiO, which can be oxidized to form particulate silica. Other methods of producing fumed silica include, for example, oxidation of SiCl4 at high temperatures or burning SiCl4 in the presence of methane or hydrogen.
Silica solutions exhibit polymerization behavior, resulting in the increase of Si—O—Si bonds and decrease of Si—OH bonds. In an aqueous medium, amorphous silica dissolves (and/or depolymerizes), forming Si(OH)4, which undergoes polymerization to form discrete particles with internal Si—O—Si bonds and external Si—OH bonds on the particle surface. Under certain conditions, the polymeric silica particles thus formed will further associate to give chains and networks comprising the individual particles.
Generally, under neutral or alkaline conditions (pH 7 or greater), the particles tend to grow in size and decrease in number, whereas under acidic conditions (pH<7), the particles have a greater tendency to aggregate to form clusters, and eventually three-dimensional networks. Salts can be present to reduce the electrostatic repulsion between particles, so that aggregation of particles will be more likely to occur Linder neutral or alkaline conditions.
The term “sol” refers to a dispersion of discrete, colloidal particles, for example, of amorphous silica in aqueous media. Under the proper conditions, sols do not gel or settle even after several years of storage, and may contain up to about 50% silica and particle sizes up to 300 nm, although particles larger than about 70 nm settle slowly. A sol can be formed, for example, by growing particles to a certain size in a weakly alkaline solution, or by addition of dilute acid to a solution of sodium silicate (e.g., Na2SiO3) with rapid mixing, until the pH drops to about 8-10, followed by removal of Na+ (e.g., by ion-exchange resin or electrodialysis). Silica sols, depending upon the type of silica, the particle size, and the nature of the particles, can be destabilized to form gels under mildly acidic to strongly acidic conditions.
The term “gel” refers to a coherent, rigid, continuous three-dimensional network of colloidal particles. Silica gels can be produced by the aggregation of colloidal silica particles (typically under acidic conditions when neutralizing salts are absent) to form a three dimensional gel microstructure. Whether a gel will form under a particular set of conditions, however, can depend on the silica properties, such as, for example, particle size and the nature of the particle surface. The term “hydrogel” refers to a gel in which the pores (spaces within the gel microstructure) are filled with water. Similarly, the term “alcogel” refers to a gel in which the pores are filled with an alcohol. When a gel is dried to form a xerogel, evaporation can result in a substantial collapse of the gel, giving a relatively high density collapsed powder. In contrast, when a gel is dried by means in which the gel microstructure is substantially preserved (e.g., supercritical drying as described in U.S. Pat. No. 3,652,214), a low density xerogel, known as an “aerogel,” is formed. Silica aerogels have very unusual and highly desirable properties such as, for example, optical transparency, extremely low density, and unprecedented low thermal conductivity. See Herrmann et al., Journal of Non-Crystalline Solids,186, 380-387 (1995).
Synthetic silicas, Such as those described above, typically are hydrophilic, owing to the silanol groups present on the surface of the silica particles. However, to make them more useful in a variety of applications, these silicas can be rendered hydrophobic by a number of different methods. One such method involves chemically treating a form of silica with silanes to replace the silanol groups with methyl groups. Alternatively, the silicas can be rendered hydrophobic by esterification with organic alcohols at high temperatures or by physical adsorption of organic polymers. In the latter case, however, such agents may remain active and can be lost from the silica in the presence of other reactive species, such as water. The corrosion resistant properties, if any, imparted to a composition by such a hydrophobic metal oxide accordingly can be degraded when the composition is exposed to such agents.
A chemically-treated fumed metal oxide can be added to a composition in addition to other rust inhibitors to promote corrosion resistance. In that respect, U.S. Pat. No. 5,098,938 describes a coating composition comprising a film-forming polymer, a corrosion inhibitor (e.g., metal chromates), and a mixture of pyrogenic and crystalline silica, which can be hydrophobic. While it has been shown that compositions similar to the ones described above provide some protection against corrosion when applied to a substrate, it would be desirable to increase the level of corrosion resistance provided by these compositions, so that they can better protect the coated substrate when subjected to adverse conditions.
Thus, a need remains for a surface coating composition that provides improved corrosion resistance when applied to various substrates. The invention seeks to provide such a surface coating composition. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.